The contribution limit to a Roth IRA in 2008 was $5000. I set up automatic deposits from my paycheck so than X dollars went to Vanguard, and in Vanguard I set it up to divide that single deposit into Y% to the Roth IRA and Z% to my money market fund. I started my job in February, not January, so my math had to take that into account, too — divide things by 11, not 12.
Despite these factors, I ended up with $4998.36 in my Roth IRA account at the end of 2008. That means I was only off by $1.64, and that was only because I couldn't use enough precision in the Vanguard percentages! I so win.
So I bought myself $1.64 worth of shares to round out the 2008 tax year to the complete $5000 limit. I am pleased with my spreadsheet math. :) Oh, and having saved that money is pleasing, too, of course. ;)